1. Field of the Invention
The present invention relates generally to the field of rotary fluid couplings and, more particularly, to a dual-seal rotary coupling having excellent sealing capabilities over a wide temperature range at low pressure as required in concentrating solar collector applications.
2. Description of the Prior Art
The prior art is replete with numerous types of rotating connectors, couplings or joints, various configurations which have been proposed for providing a reasonably fluid-tight seal at a juncture between a rotating and a stationary fluid conduit. Almost all of the prior art rotary couplings are designed for relatively constant moderate temperatures from about 0.degree. F. to 100.degree. F. In addition, most of these rotary joints are designed for relatively high pressures, i.e., above 1000 psi as in many hydraulic systems. Most of the prior art rotary couplings are sealed by the means of "O" rings or lip type seals and in most situations some leakage is tolerated. For temperatures above 300.degree. F. certain rotary joints have been devised utilizing carbon-faced, spring loaded rotary seals.
One prior art rotary fluid joint utilizing a dual seal is found in a patent to Freize et al., U.S. Pat. No. 3,889,983 issued June 17, 1975 which illustrates and describes the rotary fluid joint for high speed operations which employs a dual seal. The function of the dual seal in that rotary fluid joint is to prevent leakage of the fluid into the high speed bearings operating in conjunction with a fluid slinger which prevents the fluid from reaching the bearings. His system is not concerned with the prevention of leakage per se, any type of a low rotational speed device, or with the detection of leakage.
Because prior art rotary joints have been designed generally for high pressure operation in which the ambient temperature shifts very little during use, the seals associated with such rotary fluid couplings are not effective over a wide range of temperatures such as that required in an outdoor concentrating solar collector system wherein the ambient temperature may vary from as low as -40.degree. F. to -60.degree. F. on a cold winter night in northern Minnesota to about 400.degree. F. under full sunlight when the system is operated on a warm summer day. Also, most fluids utilized as heat transfer mechanisms for solar concentrating collectors such as Therminol 44 (trademark of the Monsanto Chemical Company of St. Louis, Mo.) are corrosive or toxic and any leakage which comes in contact with people or painted surfaces may cause a great deal of damage. Therefore, inasmuch as concentrating solar collector fields are often located on the roofs of buildings or atop parking ramps wherein people and automobiles may pass beneath the collecting field, it is imperative that no leakage of heat transfer fluid occur. Also, inasmuch as the entire piping system of a solar installation of this type typically is covered with up to four inches of thermal insulation, some method of early leak detection which can detect even minute leaks is desirable if possible.
Thus, applications such as the one described require a rotary joint which must operate without leaks in a temperature range from at least -40.degree. F. ambient to +450.degree. F. fully heated. The joint does not have to withstand high speed operation or high pressure but the material of construction, including the seals must be compatible with the selected heat transfer medium.